1. Field of the Invention
The invention is concerned with vehicles wherein the back wheels are generally powered in a usual manner by the engine thereof. More particularly, the invention is concerned with such vehicles which include a hydrostatic auxiliary front wheel drive system which operates off of the pressure developed by a hydraulic implement pump such as the pump normally used to provide power steering and implement actuation for the vehicle. More particularly, the present invention is concerned with a unique hydro-electrical priority valve and speed control circuit which assures that power to a hydraulic implement such as the power steering function of a vehicle is not so severely reduced as to be inoperable when power from the hydraulic implement pump of such a vehicle is applied to the front wheel thereof thus providing four-wheel drive to the vehicle.
2. Prior Art
The prior art teaches the use of the implement pump of a vehicle to provide a hydrostatic auxiliary front wheel drive system for such a vehicle. For example, copending application Ser. Nos. 432,968, now abandoned; 432,969, now U.S. Pat. No. 3,997,017 both filed Jan. 14, 1974; and 547,906 filed Feb. 7, 1975, now abandoned, all commonly assigned herewith, each disclose such systems, which systems include in the case of applications Ser. Nos. 432,968 and 432,969 hydraulic priority valves.
None of the above-mentioned copending applications, however, provide hydro-electric means which act positively and rapidly to ensure that the implements driven by the implement pump will continue to operate in an adequate and full manner even when the hydraulic implement pump is being used to supply power to motors which drive the front wheels of the vehicle. This can create a very serious problem, especially when such an implement pump is used, as is usual, to provide power for a power steering system. What can occur is that a vehicle which is shifted into a mode wherein it makes use of the hydrostatic auxiliary front wheel drive system thereof can suddenly experience a significant drop in pressure to the hydraulic implement or implements being operated off the implement pump, for example, in the power steering system, and this can seriously interfere with operator control of the vehicle. The prior art hydraulic system for then shutting off the front wheel drive system provide that all pressure will then be applied to the implement system but the shift in application does not always occur in a sufficiently fast and positive manner to allow the operator to always have full control of the implement system. Accordingly, the present invention is concerned with an improvement in a hydrostatic auxiliary front wheel drive system for a vehicle which improvement assures that sufficient power is always available to operate a hydraulic implement from a hydraulic implement pump as well as providing power for the hydrostatic auxiliary front wheel drive system thereof when pump developed pressure is sufficiently high.